Method of making an intraocular lens with resilient haptics

ABSTRACT

An improved intraocular lens has at least one filamentary haptic composed of a continuous matrix material interspersed with a toughening amount of discrete particles of a multistage, sequentially produced elastomeric polymer. An improved process for preparing a single-piece intraocular lens involves preparing a composite lens blank by molding pellets composed of a continuous polymeric material interspersed with particles of a multistage, sequentially produced elastomeric polymer about a central optic lens core so as to produce a single lens blank having a toughened annular region surrounding the central optic region, and then machining at least one filamentary haptic from the toughened annular region to prepare the lens.

This is a continuation of application Ser. No. 08/088,701, filed Jul. 8,1993, now abandoned, which is a divisional of application Ser. No.07/953,406, filed Sep. 29, 1992, now U.S. Pat. No. 5,282,853.

BACKGROUND OF THE INVENTION

This invention relates to an improved intraocular lens for implantationin the eye following removal of the natural lens during cataractsurgery. More specifically, it relates to such an intraocular lens withat least one filamentary support member, or "haptic", which exhibits anincreased resistance to breakage during manipulation.

The filamentary haptic of an intraocular lens is designed to facilitateinsertion of the lens into the eye and to provide stable fixation of theimplanted lens to prevent the lens from decentering. The filamentaryhaptic is attached to and extends outwardly from the periphery of theoptical lens body. Most intraocular lenses have two haptics displayed atpositions 180° apart from each other on the optical lens body.

It is critical that the haptic of the lens exhibits adequate resiliencyand significant resistance to breakage during use. Although certainhaptic materials such as polypropylene offer acceptable resistance tobreakage, other conventionally used haptic materials such aspolymethylmethacrylate (PMMA) are brittle and are frequently prone tobreakage. This problem becomes especially acute when the haptics arelathe cut from a single lens blank to prepare a one-piece lens withintegrally attached haptics. The problem of haptic breakage is a seriousone, and efforts have been made to provide the haptics with an increasedresistance to breakage.

One such effort is disclosed in U.S. Pat. No. 5,037,435. The '435 patentdescribes preparing intraocular lenses with haptics exhibiting highfracture toughness. The haptics are composed of a polymer matrix withfrom 0.1 to 0.5 percent by weight of dispersed, solid particles of aninorganic, biocompatible material. The inorganic materials disclosedinclude titanium dioxide, fumed silica, barium sulfate and copperphthalocyanate. Organic, elastomeric materials such as polybutadiene arealso described. Unfortunately, the haptics of these intraocular lensesfail to exhibit significantly increased fracture toughness because theparticle concentration in the matrix is too low to have any beneficialimpact, and the inorganic materials disclosed do not impart desiredtoughness properties regardless of concentration. In addition, theincompatibility of conventional elastomeric materials, such aspolybutadiene, with a rigid polymer matrix such as PMMA, preventsincreasing the concentration of conventional elastomeric particles to anamount necessary to improve breakage resistance without diminishing thephysical and mechanical properties of the rigid polymer matrix.

In view of the deficiencies of the prior art, it would be desirable tofabricate an improved intraocular lens with filamentary support hapticsthat exhibit an enhanced ability to withstand breakage during routinehandling.

SUMMARY OF THE INVENTION

The invention is an improved intraocular lens of the type having acentral lens body and at least one filamentary haptic attached to andextending outwardly from the periphery of the lens body. The improvementto the intraocular lens relates to the filamentary haptic, which forthis invention is composed of a continuous matrix material interspersedwith a toughening amount of discrete particles of a multistage,sequentially-produced elastomeric polymer.

In another aspect, the invention is an improved process for making aone-piece intraocular lens. A one-piece intraocular lens is made usingthis process by machining a single lens blank to form a central lensbody and at least one filamentary haptic integrally attached to andextending outwardly from the periphery of the lens body. The improvementin the process relates to first making a lens blank by molding pelletsof the appropriate composition about a cylindrical optic lens core. Thepellets are composed of a continuous polymer matrix interspersed with atoughening amount of discrete particles of a multistage,sequentially-produced elastomeric polymer. This molding operationresults in an integral lens blank having a toughened annular regionsurrounding a central optic region. The lens blank can then be machinedto form at least one filamentary haptic from the toughened annularregion in which the filamentary haptic is integrally attached to andextends outwardly from the periphery of a central lens body.

A filamentary haptic of the improved intraocular lens of this inventionexhibits surprisingly dramatic resistance to breakage under adversehandling conditions. The process for making this improved lens issurprisingly straightforward and requires only conventional processingequipment. The resistance to breakage is achieved without the loss ofthe physical or mechanical integrity of the haptic, or any otherproperty which is necessary for proper functioning and use of thehaptic. In addition, the properties of the optic lens body remainunchanged.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an individual particle of themultistage, sequentially-produced elastomeric polymer which is dispersedthroughout the matrix material of the filamentary haptic of anintraocular lens of this invention.

FIG. 2 is photomicrograph illustration at a magnification of 25,000×showing the morphology of the filamentary haptic.

FIG. 3 is a perspective view of a brittleness tester used fordetermining the resistance to breakage of the haptic of an intraocularlens.

FIG. 4 is a top plan view of a one piece intraocular lens showing thelocation and direction of rotation used in brittleness testing.

FIG. 5 is a top plan view of a mold chase used to mold the toughenedresin around optic cores.

FIG. 6 is a top plan view of a composite blank containing toughened PMMAresin around an optic core.

DETAILED DESCRIPTION OF THE INVENTION

The multistage, sequentially-produced elastomeric polymer particles arecritical for imparting the fracture resistance to the matrix material ofthe filamentary haptic. Processes for preparing such elastomericpolymers are well known, and described, for example, in U.S. Pat. No.3,793,402, incorporated by reference herein. The overall bulk propertiesof the polymer particles are such that the elastomeric stages of theparticles have a glass transition temperature (Tg) below roomtemperature and exhibit a sufficiently high molecular weight or areadequately crosslinked to achieve solid, rubber-like properties. Anadditional important factor in the bulk properties of the particles isthat the particles must be compatible or made to be compatible with thematrix material from which the haptic is composed. Unlike conventionalelastomeric particles, this compatibility can be achieved when theparticles are made using the multistage, sequential process.

FIG. 1 illustrates an elastomeric polymer particle used for impartingbreakage resistance to the haptic matrix material. As can be seen fromFIG. 1, the multistage, sequential production of the polymer yields apolymer particle that has three substantially discrete layers ofpolymeric materials exhibiting different properties.

The inner "core" layer, denoted in FIG. 1 as Component A, is made in thefirst stage of the production of the sequentially-produced elastomericpolymer. It should be nonelastomeric and glassy. It must have a Tg.greater than room temperature, preferably 60° C. or greater, and can becomposed of a polymer of an ester of acrylic or methacrylic acid, whichpolymer is crosslinked to provide its desired properties. The preferredpolymer in the core layer is crosslinked PMMA.

Surrounding the core component of the polymer particle is asequentially-produced intermediate layer, Component B, and a separateand distinct outer layer, or "shell", Component C. Component B is anelastomeric polymer, preferably composed of a polymer of an alkylacrylate, such as butyl acrylate. Alternatively, it can be composed ofbutadiene or substituted butadiene (substituted with, for example,isoprene, chloroprene and 2,3 dimethylbutadiene). The elastomers of thisstage are those that have a Tg of 25° C. or less. Preferred are thoseelastomers having a Tg less than 10° C., and most preferred are thoseelastomers having a Tg less than -10° C. Component B imparts the bulkelastomeric properties to the polymeric particles. Component C is arelatively hard polymer similar to the polymer of the core component ofthe particle. As used herein, a "hard" polymer refers to a glassypolymer which has a Tg above room temperature, preferrably 50° C. orhigher. The material from which Component C is made is preferablycrosslinked PMMA, but critically, it must be made up of a material whichis compatible with the matrix material from which the haptic iscomposed. For purposes of this invention, the material from whichComponent C is made is "compatible" with the haptic matrix material ifComponent C has a chemical composition similar to the composition of thehaptic matrix.

The finely divided, discrete elastomeric polymer particles should be ofa particle size in the submicron range. If the particles aresubstantially larger than submicron size, then the particles may have atendency to agglomerate and therefore create nonuniformity in theproperties of the filamentary haptic. Advantageously, the particle sizeranges from about 100 to about 300 nanometers (nm), preferably fromabout 160 to about 280 nm.

The continuous matrix material from which the haptic is made ispreferably polymeric. The most preferred matrix material is composed ofcrosslinked PMMA, although other materials such as copolymers of methylmethacrylate and other biocompatible polymers can be used.

The amount of polymer particles necessary to provide significanttoughening to the filamentary haptic can generally range from about 5 toabout 65 percent of the weight of the haptic, preferably from about 35to about 45 percent. This relatively high solids loading is possiblebecause of the compatibility of the multi-stage, sequentially-producedparticles with the polymer matrix. If the amount of particles is lessthan 5 percent, then the likelihood of achieving a beneficial effect onthe impact resistance of the haptic is small. If the amount oftoughening particles is greater than about 65 percent, then there is astrong possibility that the overall physical and mechanical propertiesof the haptic may be compromised.

The most preferred formulation for the preparation of the filamentaryhaptic is commercially available in the form of molding pellets (oftenreferred to as "molding powder"), which consists of individual pelletsthat are composed of the matrix material interspersed with a tougheningamount of the discrete particles of the multistage,sequentially-produced elastomeric polymer. A particularly preferredmolding powder resin from which the haptics can be made is Plexiglas®DR® acrylic molding pellets.

FIG. 2 represents a photomicrograph of a toughened filamentary haptic ofthe improved intraocular lens of this invention. The photomicrographclearly depicts discrete elastomeric particles, which in this case havebeen made using the multistage process, interspersed within a continuousmatrix. The outer shell layer of each of the discrete particles is welldefined by the dark ring surrounding the periphery of each of theparticles. The shell layer prevents the discrete particles fromagglomerating even at relatively high concentrations, so that uniformlyimproved haptic performance can be achieved.

The filamentary haptic can be attached to the optic lens body of theintraocular lens through chemical, thermal or other known physicalprocesses. Preferably, two haptics are attached to the periphery of theoptic lens body at diametrically opposed positions on the optic.

Chemically, the toughened haptics can be attached to the optic lens bodyby a copolymerization process. This process involves placing an opticcore in a suitable tubular mold, pouring into the mold a polymerizablecomposition suitable for preparing the matrix material of the haptic,which composition contains the toughening particles, and thenpolymerizing the curable resin in the mold to prepare an optic lensblank having a toughened annular region from which the haptic memberscan be lathe cut. Ideally, the annular region configured about the opticcore is colored by adding a suitable dye into the matrix material beforepolymerization. See, for example, U.S. Pat. Nos. 5,089,180 and4,961,746.

Alternatively, the filamentary haptics can be attached to the optic lensbody by conventional physical means, for example, by staking. Bondingcan also be achieved by conventional solvent welding processes.

The preferred process for attaching the filamentary haptic to the lensbody of the intraocular lens is a process in which the haptics areintegrally attached to the lens body in a one-piece intraocular lensconfiguration. This processing can be most readily achieved either bycompression molding or injection molding. In fact, the desired hapticcomposition lends itself perfectly to such processing because of theavailability of molding powder in the form of pellets which are composedof the toughened elastomeric polymer particles interspersed in thedesired polymeric matrix of the haptic.

In the compression molding process, the optic core, or optical lensbody, is placed in a suitable mold configured to allow for the moldingof the annular haptic region about the central optic core. Once theoptic core is placed in the mold, the molding powder pellets of thedesired composition, for example, Plexiglas® DR® acrylic moldingpellets, are placed in the mold completely surrounding the optic core.The mold is then heated to a temperature sufficient to soften thepellets, and then pressure is applied by compression in the mold tobring the optic core and the softened pellets into contact for properfusion and shaping of the composite lens blank. Compression also inducesnecessary degassing of pockets of air which form between individualpellets during the fusing process. Following compression at an adequatepressure for a suitable period of time, the mold is released and thetemperature of the prepared composite blank is lowered. The compositeblank can then be machined on a lathe to fabricate the filamentaryhaptics from the toughened annular region surrounding the optic core ofthe lens blank.

Another method for preparing a composite lens blank from which thehaptics can be machined for the preparation of an improved one-pieceintraocular lens of this invention would be the use of conventionalinjection molding processes well known in the art. Similar to theprocess scheme for compression molding, the injection molding processwould utilize molding pellets composed of the desired matrix material ofthe haptics toughened with appropriate multistage, sequentially producedpolymeric particles. In one embodiment, the pellets can be injectionmolded to form the configuration of an annular ring, or "donut", whichcan then be placed over an optical core rod which would form the lensbody of the intraocular lens. By application of heat and pressure, thetoughened donut could be fused to the optic rod for the preparation ofthe lens blank similar in configuration to the lens blank made by thecompression molding process described above.

As used herein, the term "pellets" is used expansively to refer to notonly traditional pellets but also derivatives of pellets which can beformed using conventional processing techniques. For example, moldingpellets can be milled or pulverized to make a fine, powder-likesubstance. For the purposes of this invention, such a substance or anycolorable imitation thereof would still be considered "pellets".

Other means for integrally attaching the filamentary haptic to the lensbody of the one-piece intraocular lens are well within the scope ofthose skilled in this art. For example, in addition to the methodsillustrated above for such attachment, the molding powder formulationfor the preparation of the haptics can be extruded about an optic corerod through a conventional wire extrusion dye used for the preparationof coated wires and coated tubular members.

In another embodiment of this invention, the haptics are tinted orcolored to provide a better visual aid during surgery by incorporationof a suitable dye into the haptic composition. Although many means foraccomplishing the incorporation of a dye into the haptic compositiondescribed herein can be envisioned, one such method would involvecompounding the dye into the desired molding pellet formation for thehaptic composition by conventional extrusion techniques. Similarly, thedye can be incorporated into the proper molding composition in acompression or injection mold.

The following examples are designed to illustrate the preferredembodiments of this invention, and should not be construed in any way tolimit the full breadth and scope of that which is defined as theinvention in the appended claims.

BRITTLENESS TEST METHOD

FIG. 3 shows a brittleness tester 10 used for determining the resistanceto breakage of the haptic of an intraocular lens. The lens 20 is placedin a fixture 11 that holds the lens securely by the lens body of thelens. As shown more clearly in FIG. 4, the optic-haptic junction of thelens is placed at the center of rotation 13 on the tester. The testeroperates by moving the rotatable pin 14 against one of the haptics 24.This forces the haptic to rotate around the center of rotation 13 in aclockwise direction. The speed of rotation can be controlled from acomputer interface 15, and can be varied up to 900 degrees per secondusing stepper motor 16. The maximum rotation angle is 140°. The encoder17 accurately measures the rotation angle and feeds the informationthrough the computer interface. The rotating arm 18 and strain gauge 19allow the accurate measurement of force necessary to move the haptic.The individual haptic thickness and width measurements are entered intoa computer in order to calculate stress from the force measurements. Astress-angle curve obtained through brittleness testing is similar to atypical stress strain curve obtained by conventional mechanical testing.Stress strain testing gives an indication of the strength of a materialand also its toughness. Toughness is defined as the area under thestress-strain curve or stress-angle curve. The brittleness test is,therefore, an effective tool to evaluate haptic performance againstbreakage.

EXAMPLE 1 Compression Molded Lens

Referring now to FIGS. 5 and 6 in combination, an intraocular lens withtoughened, impact-modified haptics is fabricated from a composite rodblank 60 containing an annular region 50 of molded Plexiglas® DR®acrylic resin centered about, and bonded to, a PMMA optic core rod 40. Anumber of composite rod blanks can be produced simultaneously bycompression molding of the DR® resin around the optic core rods in amold chase 30. The mold chase has a mold plate 32 containing a pluralityof cavities for the optic core rods, and an open area 34. DR® pellets50a are loaded in the open area surrounding the optic core rods. Themold chase is then heated at 210°±5° C. for 10 minutes in a press priorto molding at a pressure of 10,000 lbs. The composite blanks, each about16 mm in diameter, are then machined from the molded sheet. The lens isthen fabricated from the molded composite blank using standard lensmanufacturing processes. The brittleness test results of the impactmodified haptics and a competitive lens, Control #1, are presented inTable 1.

The lenses tested as Control #1 are made from a proprietary materialformulated with a low concentration of biocompatible particulates in thehaptics to supposedly give increased haptic toughness.

                  TABLE 1                                                         ______________________________________                                        Brittleness Test Results                                                                                     Max. Angle                                                                            Frac-                                                                 or Angle at                                                                           ture                                   Ex. No. Sample ID Model        Break deg                                                                             Rate                                   ______________________________________                                        Control #1                                                                            Intraocular                                                                             ORC Flexeon ®                                                                          37      100                                            Lens      C410                                                        #1      Intraocular                                                                             Plexiglas ® DR ®                                                                   >140    0                                              Lens      acrylic resin                                                                 toughened haptics                                           ______________________________________                                    

As shown from the data in Table 1, lenses from Control #1 do not showimproved resistance to haptic fracture. In contrast, the data indicateno haptic breakage upon brittleness testing for the lens of thisinvention having the toughened haptic material. Thus, the resultsdemonstrate the ductile and flexible nature of the haptics toughenedwith the DR® acrylic resin.

EXAMPLE 2 Injection Molded Lens

An intraocular lens with impact modified colored haptics is fabricatedfrom a composite blank containing an annular region of molded Plexiglas®DR® acrylic resin and colored dye centered about, and molded to, a PMMAoptic core. In this example, D&C Violet #2 dye is pre-mixed in thePlexiglas® DR® resin at 0.15% w/w. The composite blanks are produced byinjection molding of the pre-mixed violet DR® resin around the optic rodin a mold. The mold consists of a single cavity Round Mate style toolwith a pocket to insert the optic rod. The optic rod is preheated to220° C. before placing in the mold. The molding conditions used arelisted below.

Resin temperature: 260° C.

Mold temperature: 105° C.

Injection pressure: 23,000 psi

Injection speed: 6.86 in³ /sec

The lens is then fabricated from the injection molded composite blankusing standard lens manufacturing processes. The brittleness testresults for this lens, as well as a competitive lens denominated asControl #1 and a conventional lens denominated as Control #2, arepresented in Table 2.

                  TABLE 2                                                         ______________________________________                                        Brittleness Test Results                                                                                     Max. Angle                                                                            Frac-                                                                 or Angle at                                                                           ture                                   Ex. No. Sample ID Model        Break deg                                                                             Rate                                   ______________________________________                                        Control #1                                                                            Intraocular                                                                             ORC Flexeon ®                                                                          37      100                                            Lens      C410                                                        Control #2                                                                            Intraocular                                                                             Iolab UV grade                                                                             56      100                                            Lens      PMMA 8590B                                                  #2      Intraocular                                                                             Violet Plexiglas ®                                                                     >140    5                                              Lens      DR ® acrylic resin                                                        toughened haptics                                           ______________________________________                                    

As can be seen from the data in Table 2, lenses made from Control #2,which do not have toughened haptics, show typical haptic fracturecharacteristics for untoughened PMMA lenses. The lenses tested asControl #1 are made from a proprietary material formulated with a lowconcentration of biocompatible particulates in the haptics to supposedlygive increased toughness. Lenses from Control #1 do not show improvedresistance to haptic fracture as compared to the untoughened PMMAlenses. In contrast, the lenses of this invention made with hapticstoughened with the DR® resin show a dramatically reduced fracture ratein comparison to those of the controls.

We claim:
 1. A process for preparing a one-piece intraocular lens madeby machining a single lens blank to form a central lens body and atleast one filamentary haptic integrally attached to and extendingoutwardly from the periphery of said lens body, the improvement in theprocess wherein said intraocular lens is made by first molding pelletscomprised of a continuous polymeric matrix interspersed with atoughening amount of discrete particles of a multistage, sequentiallyproduced elastomeric polymer, about a cylindrical optic lens core so asto form said lens blank having a toughened annular region surrounding acentral optic region, and wherein each of said discrete particles has aninner core layer of a glassy polymer, an intermediate layer of anelastomeric polymer, and an outer shell layer of a glassy polymer whichis compatible with said continuous polymeric matrix, and then machiningsaid filamentary haptic from said toughened annular region.
 2. Theprocess of claim 1 wherein said pellets are molded about said optic lenscore by compression molding or injection molding.
 3. The process ofclaim 1 wherein the core layer is composed of a polymer of acrylic ormethacrylic acid, said polymer being crosslinked.
 4. The process ofclaim 3 wherein the core layer is composed of crosslinked PMMA.
 5. Theprocess of claim 4 wherein the intermediate layer is composed of apolymer of an alkyl acrylate, or substituted or unsubstituted butadiene.6. The process of claim 5 wherein the intermediate layer is composed ofa polymer of butyl acrylate.
 7. The process of claim 6 wherein the outershell layer is composed of crosslinked PMMA.
 8. The process of claim 7wherein the size of each of the discrete particles ranges from about 100to about 300 nm.
 9. The process of claim 8 wherein the size ranges fromabout 160 to about 280 nm.
 10. The process of claim 9 wherein thepolymeric matrix is crosslinked PMMA.
 11. The process of claim 10wherein the toughening amount of discrete particles interspersed in thepolymeric matrix is between about 5 to about 65 percent of the weight ofthe haptic.
 12. The process of claim 11 wherein the toughening amount ofdiscrete particles interspersed in the polymeric matrix is between about35 to about 45 percent of the weight of the haptic.